The solder levelling means is intended to provide a substantially uniform thickness of solder on the exposed copper and to unblock any holes through the board which might otherwise be blocked by solder. The levelling means may be arranged to direct hot liquid at the board as it is withdrawn from the solder. However, it is presently preferred to direct hot air at the board by means of so called "air knives" for levelling the solder as the board is withdrawn from the solder bath.
Such a solder leveller is described in UK Patent Specification No. 1,457,325 and equivalent U.S. Patent Specification No. 3,865,298 (U.S. Atomic Energy Commission). This prior leveller includes means for deflecting hot air from the air knives away from the surface of the molten solder in the solder bath. These deflection means can in practice be dispensed with, particularly where the molten solder in the bath is continuously recirculated whereby any flux and/or dross (oxidized solder) is carried by the flow of the molten solder away from the area of the surface of the molten solder through which the board is to be inserted.
When a board is being treated in such a recirculating solder leveller, it may be displaced from its preferably central position in the solder bath to the extent that it is scratched on withdrawal from the bath. Scratching is likely to result in the board being reject. Such scratching is believed to result from contact with the walls of the solder bath.
The usual manner of supporting a board for insertion in the bath is from above. This can lead to the possibility of the board being deflected by the flow of solder to one side of the bath and contacting the corresponding side wall whereby on withdrawal of the board it scrapes along the side wall and becomes scratched.
Accordingly, in my UK Patent No. 2,151,528 (which has an equivalent U.S. Pat. No. 4,599,966) I describe and claim a solder leveller including means for recirculating molten solder to and from the solder bath which comprises parallel spaced apart side walls, end walls and a bottom wall, the recirculating means comprising a plurality of side wall ducts having side wall ports therein one above another to direct a flow of solder against opposite sides of a board inserted in the bath, an elongate bottom wall duct extending along the bottom wall of the bath, the bottom wall duct having ports communicating with the lower ends of the side wall ducts, and means for pumping solder through the elongate bottom wall duct, the side wall ducts and the bath, in such manner that the flow of solder in the solder bath biasses a board inserted therein away from side walls of the solder bath.
However, although this leveller has proved successful, in practice, it is normally necessary to augment the effect of the side wall ducts on the centring of the board in the solder bath with guides for guiding the edges of the board, if only to ensure that as the board is initially lowered into the solder bath it is centred. These guides can cause problems. In particular, they interfere with the deposit of solder onto the edge regions of circuit boards.
Irregularly shaped boards are awkward to process because part of their edge portions are located by the guides whilst other parts do not extend as far as the guides and are not located. These problems are aggravated with smaller boards.
Small boards provide an additional problem in that they may have no spare surface area at which a clamp or other holding means active at the top edge of the board can hold them.